A former Credit Suisse trader admitted Thursday he cooked the books to boost his year-end bonus. It’s all part of the much-wider subprime mortgage mess that has led to worldwide financial calamity. (Fabrice Coffrini/AFP/Getty Images)

Looking to fatten their year-end bonuses, a trio of Credit Suisse execs cooked the books as part of a "colossal fraud" tied to the subprime mortgage crisis, authorities said Wednesday.

Kareem Serageldin, the bank's ex-head of structured credit, was to line his pockets with an extra $7 million on top of his $280,000 salary in 2007, federal prosecutors said.

But Credit Suisse withheld $5.2 million of his pay after it got wind of the fraud.

Investigators said because of the phony record-keeping by Serageldin and his two cronies, David Higgs and Salmaan Siddiqui, the financial firm was forced to cut the value of its 2007 results by $540 million.

"It is a tale of greed run amok," U.S. Attorney Preet Bharara said. "They papered over more than a half billion dollars in subprime mortgage-related losses to secure for themselves a big payday at the same time that many people were losing their homes and their jobs."

Serageldin, 38, an American living in England, wasn't in custody Wednesday, and there were no immediate plans to seek his extradition on charges of conspiracy, falsifying records and wire fraud.

If convicted, he faces up to 45 years behind bars.

Higgs, 42, a former London-based managing director, and Siddiqui, 31, an ex-VP who worked in Manhattan, pleaded guilty to fraud and conspiracy charges.

They admitted to inflating figures in order to advance in the firm and boost their bonuses — all while the housing market collapsed.

"I was directed by my bosses and my boss's bosses," Siddiqui said.

Both face up to five years behind bars and are cooperating in the federal probe.

In a separate action, the SEC filed civil charges against the trio and a fourth trader, who wasn't criminally charged.

The case — focused on shady dealings at the bank between 2007 and 2008 — is one of only a handful brought over charges linked to the subprime mortgage market.